1. Field of the Disclosure
Embodiments disclosed herein relate generally to the production of tertiary alkyl ethers. More specifically, embodiments disclosed herein relate to the production of tertiary amyl ethyl ether from feedstocks that contain nitrile impurities, such as acetonitrile and propionitrile.
2. Background
Etherification reactions are the reaction of olefins, such as isobutylene and isoamylenes, or other olefinic compounds, with an alcohol to form the corresponding ether. For example, isobutylene may be reacted with ethanol to form methyl tertiary butyl ether (MTBE).
Etherification reactions may provide for introduction of oxygen into gasoline to produce cleaner burning reformulated fuels. For example, ethers that may be blended with gasoline include MTBE, ethyl tertiary butyl ether (ETBE), tertiary amyl methyl ether (TAME), tertiary amyl ethyl ether (TAEE), and tertiary hexyl methyl ether (THEME), among others. Ethers not only introduce oxygen into gasoline, but they also result in an increased octane rating, may improve the anti-knocking characteristics of the motor fuels, and may reduce the concentration of detrimental components in the exhaust gases.
Etherification reactions may also provide for high purity olefin feedstocks. For example, following etherification of a mixed C4 feedstock to form MTBE, the resulting mixture containing ether may be separated to recover the ether. The MTBE may then be cracked to form alcohol and isobutylene, which may be separated to result in a high purity isobutylene. The resulting high purity isoolefins may be used, for example, in polymerization processes which require a high purity feedstock.
The etherification process typically uses strongly acidic ion exchange resins as etherification catalysts, such as strongly acidic organic polymers. As an isobutylene or isoamylenes molecule meets alcohol at an active catalyst site, the reaction between the olefin and the alcohol takes place, rapidly forming ether.
The activity of the catalyst for etherification reactions is a function of the acid loading or capacity of the resin. This functionality is not linear: a loss of 20% of acid sites on the catalyst results in approximately 50% loss of activity for the conversion to ether. It is therefore important to minimize the deactivation of the catalyst. The loss of catalytic activity may be caused by the adsorption of basic compounds or metal ions, the blockage of the active sites by polymeric products, reaction with acetylenic compounds in the feed, or splitting off the resin's functional groups due to long term operation at temperatures above 240° F. The latter two causes are affected by operating conditions of the etherification reactor. The major source of lost activity is typically from poisons entering with the feedstocks to the unit. Poisons to the catalyst include basic compounds, such as ammonia, amines, caustic soda, and nitrites, for example. In particular, nitiriles, such as acetonitrile (ACN) and propionitrile (PN), have been found to deactivate the catalyst. Some studies have shown that nitrites are converted to basic nitrogen compounds which are catalyst poisons, such as described in U.S. Pat. No. 5,675,043. Regardless of the actual poison, nitrites or derivatives thereof are generally not desired to be fed to the etherification reaction vessel due to the potential for catalyst deactivation.
In refinery applications, the largest source of hydrocarbon feedstock containing isoolefins is the stream from the fluidized catalytic cracking unit (FCCU). Some C4s and C5s are also obtained from fluid or delayed cokers. Nitriles formed in these units enter the etherification process with the hydrocarbon feed stream. The amount of nitriles in the feed varies with the severity of the catalytic cracker operation, crude source, and catalyst used in the FCCU.
Propionitrile has been found to be a particular problem in the C5 stream. Unlike other feed poisons which may deactivate the catalyst in a plug flow fashion through the catalyst bed, nitriles' deactivation mechanism is not immediate. Rather, it results in a diffused deactivation throughout the entire bed. In order to obtain adequate run lengths with the catalyst, it is important to minimize the contact of the catalyst with poisons, including nitrites.
Etherification processes include fixed bed reaction systems, catalytic distillation systems, and combinations of fixed bed reactors and catalytic distillation, among others. Such processes are described in U.S. Pat. Nos. 5,238,541, 5,489,719, 5,491,267, 6,037,502, 5,446,231, 5,536,886, 6,583,325, 5,166,454, and 5,188,725, among others.
Many of the above listed patents describing etherification processes recognize the need to minimize contact between etherification catalysts and nitrites. Several processes have been proposed to remove nitrites from the etherification unit feed. For example, U.S. Pat. Nos. 5,569,790, 5,672,772, 5,847,230, 6,037,502, 6,118,037, 6,278,029 disclose etherification processes in which the nitrites are removed from C4 and C5 hydrocarbon streams by washing the hydrocarbon stream with water and/or alcohols to extract the nitrites into the water phase. However, as noted in U.S. Pat. Nos. 5,446,231 and 6,019,887, water washing may be inefficient at extracting nitrites, especially propionitrile.
Other processes to protect etherification catalysts from deactivation with nitrites include solvent extraction, selective hydrogenation of dienes, hydrolysis, hydrogenation of nitrogen and sulfur containing compounds, reactive guard beds, nitrogen removal units, and adsorption, each prior to etherification. Other processes include integrating catalysts for use in FCC and etherification processes, or using other processes which provide for catalyst beds that may be regenerated, such as esterification, followed by subsequent reaction stages. Examples of these processes may be found in, for example, U.S. Pat. Nos. 5,015,782, 5,166,454, 5,188,725, 5,352,848, 5,414,183, 5,491,267, 6,019,887, 6,118,037, and 7,025,872. With regard to guard beds, however, as noted in U.S. Pat. Nos. 5,292,993 and 6,197,163, catalyst poison removal may be inefficient, or some catalyst poisons may be able to pass through a conventional guard bed.
Another process to protect etherification catalysts from deactivation with nitrites includes the azeotropic distillation of the reactants, hydrocarbons and alcohols, prior to the etherification reactor. For example, methanol azeotropic distillation is described in U.S. Pat. Nos. 5,238,541, 5,292,993, 5,453,550, 5,446,231, and 6,197,163. For example, in U.S. Pat. No. 5,238,541, the hydrocarbon stream containing isoolefins is contacted with alcohol and the mixture is distilled. As a result of the azeotropes formed in the mixture, the hydrocarbons may be removed overhead with some alcohol while the nitrites, alcohol, and heavier hydrocarbons are removed in the bottoms, resulting in a hydrocarbon feedstock, taken as overheads, substantially free of nitrites. The alcohol/hydrocarbon mixture may then be used as a direct feed to the etherification unit as alcohol is a reactant in the process.
In U.S. Pat. No. 5,446,231 ('231), incorporated herein by reference, a hydrocarbon stream containing nitrites is contacted with a methanol-water mixture to extract the nitrites. The resulting water-methanol-nitriles mixture is then distilled to recover water as a bottoms and nitrile-contaminated methanol as overheads. The hydrocarbons, having decreased nitrile content, may then be fed to an etherification reactor.
In one embodiment of '231, the methanol-nitrile mixture is fed to a catalytic distillation reactor at a point below the catalyst zone. The methanol forms an azeotrope with the hydrocarbons and is distilled into the catalyst zone. The nitriles do not enter the methanol-hydrocarbon azeotrope and remain in the ether product, collected as the bottoms from the catalytic distillation reactor. The net effect is to hold the nitrites out of contact with the cation resin catalyst and to return them to the stream following conversion to the higher octane ether, thus extending catalyst life.
In a separate embodiment, '231 discloses that ethanol-water blends may be used effectively for the extraction of propionitrile from a C5 fraction. '231 further teaches that the propionitrile must be removed from the ethanol prior to recycle to the catalytic reactor in order to protect the catalyst. With use of ethanol-water mixtures, '231 indicates that hydrogenation of the nitrites to amines would be appropriate. Thus, while nitrile-contaminated methanol may be fed to the distillation column reactor, '231 indicates that nitrile-contaminated ethanol should not be fed to the distillation column reactor.
In general, as discussed above, these and other references teach that it is undesirable to feed nitrites to the etherification reaction vessel due to heightened potential for catalyst deactivation. To prevent catalyst deactivation due to nitrites, many of the above described processes may include a significant number of reactors and separators (a high piece count), translating to higher capital costs, increased complexity to the overall process, and increased operating costs (energy, raw materials, purification); may result in decreased olefin feed concentration; and/or, as described above, may be inefficient at deactivating contaminant/propionitrile removal.
Accordingly, there exists a need for a process to produce ethers from C4 to C6 hydrocarbon streams, where the process may have a reduced piece count, may effectively avoid deactivation of the etherification catalysts, and/or may provide an economic alternative to prior art etherification processes.